How do I make the camera's depth of field affect an imported image?
The image contains no depth information, to blender it is just a plane, so you won't be able to have any selective defocusing based on the distance to the camera.
If you want to blur parts of it selectively, you need to create some kind of depth map: a black and white image in which the close areas are dark and the far ones are bright.
Then use that image in the compositor. Feed the original image to a defocus node and the fake &quot;depth map&quot; as the Z information. You can use a color ramp to change the values to determine what areas get blurred.




Read:
How can I get a Depth of field render pass?
Blender (Cycles) Manual DOF